An example of a pneumatic spring (sometimes called an air spring because of the common use of air as the medium that fills the spring interior) comprises a top closure, a bottom closure, and a generally cylindrical flexible sleeve or bellows whose opposite ends are closed by the respective closures. The top closure is attached to an underbody component of a vehicle, such as a chassis frame rail or a structural floor rail that may be integrated with the vehicle body. The bottom closure is attached to a suspended component, such as an axle or bogie, containing wheels on which the vehicle travels. While various constructions for sleeves and bellows are known, they share a common characteristic of being gas-impermeable for containing a gas under pressure and retaining their general shape while contracting and expanding with motion of the closures toward and away from each other as the unsprung mass moves toward and away from the sprung mass.
Each closure is sealed to the respective end of the sleeve or bellows in a suitably appropriate fluid-tight manner to create a fluid-tight chamber that can expand and contract in length. The top closure typically contains a port through which a gas, such as air, can be introduced into or vented from the closed interior of the spring to set a suitable interior pressure.
It is known to fabricate closures from various materials that include metals and synthetics.